Current AlGaInN transistor devices include bipolar junction transistors (BJT) and heterojunction bipolar transistors (HBT). When designing BJTs, to have low power consumption and achieve a high current gain, the forward injection current in BJTs may be maximized while the backward injection current may be minimized. However, this design requires a trade-off between low base resistance and large forward injection current, both of which are desirable for optimum transistor performance.
Using nitride HBT that includes a wider-bandgap material for the emitter region than for the base region may minimize this performance tradeoff. However, both n-p-n and p-n-p HBTs have significant design limitations due to fundamental material properties. The n-p-n HBTs have several issues. The “memory effect” of Mg in the growth reactor when forming the middle p-doped base region makes it difficult to achieve good n-type doping in the upper n-layer. It is also difficult to achieve high p-doping levels in the very thin p-type base layers resulting in high base region resistance. Etching damage when accessing the buried layer causes high contact resistances when forming electrodes. Unlike n-p-n transistors, p-n-p nitride HBTs can have highly conductive base layers, but the choice of bandgap for the emitter and collector is limited by the difficulty of p-doping high Al-containing AlGaN alloys. This limitation severely impacts the accessible performance enhancement of nitride p-n-p HBTs.
In light of the shortcomings of the existing nitride BJTs and HBTs, there is a need for improved p-n-p transistor structures and materials that enable high p-doping of high-bandgap emitter and collector regions.